Thin layers of material comprising Cu(In,Ga)Se, i.e. CIGS, are known to exhibit the highest photovoltaic conversion efficiency of any thin film material for a photovoltaic device (19.5%). See K. Ramanathan et al., “Properties of High-Efficiency CIGS Thin-Film Solar Cells,” 31st IEEE Photovoltaics Specialists Conference and Exhibition, Lake Buena Vista, Fla., Jan. 3-7, 2005; and D. E. Tarrant et al., “CIS thin film development and product status at Shell Solar, May 2003,” Proc. of 3rd WCPEC, Osaka, Japan, May 2003. Similar progress has been reported in the manufacturing area, where the efficiency of champion modules has exceeded 13% with yield above 80%. See M. Contreras et al., “High Efficiency Cu(In,Ga)Se2-Based Solar Cells: Processing of Novel Absorber Structures,” Proc. of 1st WCPEC, Hi., Dec. 5-9, 1994. Consequently, CIGS is considered by many in the art to be an attractive material for use in the manufacture of thin film photovoltaic panels.
In a typical solar cell module, as shown in FIG. 1A, a CIGS layer 106 is grown to a thickness of about 2 μm on an underlying metal layer 104 comprised of molybdenum (Mo). Mo provides good ohmic contact and has a similar thermal coefficient of expansion as CIGS, which can be important for the elevated temperature of CIGS processing. The Mo is usually deposited on a substrate material 102 which may be, for example, glass, stainless steel, or plastic. A range of concentrations of Cu, In, Ga and Se can be used in the CIGS layer, and sometimes the Ga is absent or S may be added. Accordingly, CIGS is used herein in its broadest possible meaning in the context of similar or related films. Moreover, other materials may be used in thin film photovoltaic modules, either individually or in combination, and either alternatively or additionally. These include CdTe, amorphous silicon (a:Si) and micro- or nano-crystal silicon (μc:Si). See generally, Y. Hamakawa (ed.), “Thin-Film Solar Cells,” Chapter 10 (2004), the contents of which are incorporated herein by reference.
While the above-mentioned reported efficiencies of thin-film photovoltaic modules including CIGS are promising, there is a large gap between those numbers and actually-obtained efficiencies of known commercial photovoltaic modules containing CIGS. One problem is that laser and mechanical scribes are commonly used to pattern and form interconnects in thin-film photovoltaic modules, and these prior art processes have a number of drawbacks that limit module efficiency. For example, they create wide scribes, defects, and shunt current paths. Furthermore, they provide limited means for wiring the module in series-parallel arrangements that might reduce sensitivity to series resistance, shading losses or non-uniformity.
For these and other reasons, some have considered using lithographic patterning processes to form thin-film photovoltaic module interconnects. However, these processes would require the ability to etch Mo, and, in some cases, to do so selectively so that, for example, the etch will not induce excessive undercut of an overlying CIGS layer. The prior art literature provides scant reference to etching Mo in a CIGS solar cell, and is otherwise insufficient to solve this problem.
Moreover, it was not even known to etch CIGS in a solar cell until the invention of U.S. patent application Ser. No. 11/395,080 (AMAT-10936), the contents of which are incorporated herein by reference. While this invention dramatically advanced the state of the art of thin-film photovoltaic modules, and also mentions etching Mo, additional problems have arisen that were not seriously addressed before that invention.
For example, as shown in FIG. 1B, certain CIGS growing processes can include selenium annealing at high temperature. In such processing, a MoSe2 layer 108 is formed at the interface between the CIGS layer 106 and Mo layer 104.
When such a MoSe2 layer is formed, both the Mo layer and this additional MoSe2 layer need to be removed during processing, and ideally using an etch. Again, the prior art literature is insufficient for overcoming this newly-observed problem. For example, T. Ohmori et al., in their article entitled “pH Dependent Controlled patterning of p-MoSe2 Surfaces by In-Situ Electrochemical Scanning Tunneling Microscopy,” Langmuir, 14 (21), 6287-6290 (1998) propose using a solution of 0.05M NH3 and 0.025M KNO3 with the assistance of a high electrical field induced between an Atomic Force Microscope (AFM) tip and a MoSe2 surface. For a typical gap of 2 nm between the AFM tip and the substrate and with the reported etching threshold voltage of 0.3V, the electrical field is as high as 1.5×108 V/m which is unsuitable for application to macro-scale processes such as photovoltaic module fabrication. Likewise, S. Chandra and S. N. Sahu, in their paper entitled “Electrodeposited semiconducting molybdenum selenide films: I. Preparatory technique and structural characterization,” J. Phys. D: App. Phys., Vol. 17 (1984), pp. 2115-2123, propose an electro-deposition method of MoSe2 films. While the article implies a MoSe2 etch in basic solutions, no etch recipe is given, and in any event it does not describe a useful process for photovoltaic module fabrication.
Therefore, there remains a need in the art to overcome many of the shortcomings of the conventional processes for etching an underlying metal Mo layer in a thin-film photovoltaic device having CIGS material. The present invention aims at doing this, among other things.